Biosensors & Bioelectronics 13 (1998) 1197–1203 In situ mapping of community-level cellular response with catalytic microbiosensors 1 Serban F. Peteu a,b,* , Mark T. Widman c , R. Mark Worden a,d a Department of Chemical Engineering, Michigan State University, East Lansing, MI 48824, USA b Department of Chemistry, University of Michigan, Ann Arbor, MI 48109-1055, USA c Pharmacia and Upjohn, Technical Support and Engineering, 7000 Portage Road, Kalamazoo, MI 49001-0199, USA d Center of Microbial Ecology, Michigan State University, East Lansing, MI 48824, USA Received 11 February 1998; accepted 20 May 1998 Abstract Chemotaxis, the migration of cells in the direction of a spatial chemical gradient, is important in disease progression, microbial ecology, and bioremediation. The ability to map chemoattractant gradients and the corresponding cellular growth and motility patterns is essential to the study of chemotaxis. Microelectrodes and microbiosensors have the potential to measure chemoattractant gradients with high spatial resolution. In this study, Clark-type amperometric microelectrodes and microbiosensors were used to measure solute concentrations gradients generated by a chemotactic band of Escherichia coli in a semi-solid gel. A computerized image analysis system was used to simultaneously measure the cellular concentration profile across the chemotactic band. The experimental results compared favorably with a mathematical model of solute and cell transport in the gel. Scanning electron micrographs (SEM) of micro(bio)sensor tips taken after 6 months of use showed evidence of degradation, including adhesion of foreign particles to the glass body, the adhesion of a small gel capsule to the sensor tip, and separation of the bio-interface from the tip. A needle-type microbiosensor was constructed to better protect the tip and hence increase the ruggedness of the microbiosen- sors.  1998 Elsevier Science S.A. All rights reserved. Keywords: Microbiosensors in situ; Tip durability; Needle-type; Chemotaxis 1. Introduction This paper describes the use of microelectrodes and microbiosensors to study cellular growth and motility at the community level. Motile cells often exhibit chemo- taxis, in which the cells move in a biased random walk in the direction of a spatial gradient of a chemical subst- ance—chemoattractant (Adler, 1972; Berg and Tedesco, 1975; Harwood et al., 1990; Kato et al., 1992; Macnab, 1987; Yamamoto and Imae, 1993). As described in more detail elsewhere (Widman et al., 1996), chemotactic bands are thought to arise from cellular uptake of the chemoattractant, resulting in a spatial gradient which draws more cells into the vicinity by chemotaxis. As the cells deplete the chemoattractant, the cells follow the * Corresponding author at University of Michigan. Tel: (313) 764- 2169, Fax: (313) 647-4865, E-mail: peteu@egr.msu.edu 1 This paper was presented at the Fifth World Congress on Biosen- sors, Berlin, Germany, 3–5 June 1998. 0956-5663/98/$ - see front matter  1998 Elsevier Science S.A. All rights reserved. PII:S0956-5663(98)00070-0 gradient away from the inoculation zone into new regions. Chemotaxis is an important factor in disease progression, microbial ecology, and bioremediation. In principle, chemotaxis could be engineered into in situ bioremediation processes to direct the movement of cells through subsurface environments. In some cases, the cells exhibit chemotaxis toward the target pollutant itself. The importance of cellular motility or chemotaxis has been reported for a phenanthrene-metabolizing Pseudomonas sp. (Devare and Alexander, 1995), for Pseudomonas stutzeri KC, which degrades carbon tetra- chloride (Witt, 1994; Worden, unpublished data), and for bacteria consuming various xenobiotic chemicals (Bosma et al., 1988). A quantitative approach is needed for predicting sub- surface bacterial population distributions. Mathematical models have been developed to predict cell motility through porous media (e.g. Barton and Ford, 1997; Dillon et al., 1996). Experimental methods have been developed with which to measure cell motility and the